1. Field of the Invention
The invention relates to a telephone handset and more particularly to a leak-tolerant multifunction telephone handset earpiece for handsfree use.
2. Description of the Prior Art
Prior art mobile telephones such as the Alcatel One Touch Pocket (registered trademarks) have a casing in which there is a cavity filled with air. Transducers constituting an earpiece and a microphone are placed in the cavity, on a wall of the casing. The earpiece transducer has a diaphragm for converting an electrical signal into soundwaves. Apertures are provided in the housing, in line with the transducer diaphragm, through which the soundwaves are emitted. There is a volume of air to the rear of the diaphragm to enable it to move and emit a soundwave of sufficient amplitude.
A small gap between the earpiece and the ear of the user causes sound leakage. The acoustic impedance seen by the transducer is then significantly modified.
The mobile telephone previously described operates only in a discreet mode, i.e. with the ear of the user close to the earpiece. It is highly sensitive to variations in acoustic impedance due to a slight gap between the ear and the transducer. It is not leak-tolerant. If the user's ear is not pressed fully against the earpiece, or moves slightly away from it, the frequency response curve at the ear is greatly modified. For example, low frequencies perceived by the ear of the user are strongly attenuated in the discreet mode. This strongly degrades the perceived sound quality. Also, this mobile telephone has no handsfree or speakerphone mode.
The document EP-A-364 935 describes a telephone in which the diaphragm of the earpiece transducer is connected acoustically to the ear of the user by apertures in the telephone. The front of the diaphragm is acoustically coupled to the interior volume of the telephone.
The documents DE-2 815 051 and U.S. Pat. No. 4,160,135 describe earpieces having a connection between the front of the diaphragm and the volume to the rear of the diaphragm.
The document WO-A-98/24214 describes a leak-tolerant mobile telephone having an interior volume and a transducer. Apertures leading to the exterior are formed on the front side of the transducer, whose diaphragm faces the apertures. Other apertures connect the interior volume to the exterior.
The document WO-A-00/21330 describes a mobile telephone having a large volume of air inside the casing to the rear of the transducer. This mobile telephone is leak-tolerant in that the frequency response curve is relatively insensitive to a slight gap between the transducer and the ear. However, the dimensions of the volume of air make the casing too bulky for a mobile telephone.
The document WO-A-00/21330 also describes a leak-tolerant mobile telephone having a small volume of air to the rear of the transducer and coupled to the atmosphere to form a resonant acoustic circuit. However, this mobile telephone has a cut-off frequency band in the audible spectrum. The perceived acoustic quality is then degraded.
The document WO-A-00/21330 further describes a leak-tolerant mobile telephone having a pair of connections between the rear of the diaphragm and the exterior and a pair of connections between the front of the diaphragm and the exterior. In each pair of connections, one connection opens into a contact area for the ear of the user and another connection opens to the atmosphere at a distance from the contact area. This telephone combines a small interior volume and an acoustic short circuit due to leaks. It therefore features a cut-off at low frequencies.
The above telephone handsets are leak-tolerant in discreet mode, but have drawbacks. None of them provides a handsfree or speakerphone mode, i.e. a mode in which the user is away from the handset.
Also, the Alcatel One Touch 300 mobile telephone (registered trademarks) offers a speakerphone mode but is not leak-tolerant in discreet mode.
Thus none of the above telephones combines good leak tolerance in discreet mode with a handsfree or speakerphone mode.
There is therefore a need for a telephone handset having a handsfree mode and a leak-tolerant discreet mode.